Ethylene-based polymers are formed by various molding methods and subjected to various applications. Characteristics required of the ethylene-based polymer vary depending on the molding method and applications. For example, when a cast film is to be formed in T-die molding, there arises neck-in wherein the edge of the film is shrunk in a direction toward the center. When neck-in occurs, the film width is decreased and simultaneously the film edge is thicker than the film center, thus reducing the product yield. To minimize neck-in, an ethylene-based polymer having high melt tension relative to molecular weight should be selected. This property is also necessary for preventing sagging or rupture in blow molding or for preventing bubble tremble or rupture in inflation film.
High-pressure low-density polyethylene is used in films and hollow containers because it is superior in moldability with high melt tension to an ethylene-based polymer produced using a Ziegler catalyst. However, the high-pressure low-density polyethylene has a complicated long-chain branched structure, and is thus expected to be inferior in mechanical strength such as tensile strength, tear strength or impact resistance. For the same reason, it is expected to be inferior in high-speed film processability in T-die molding.
The ethylene-based polymer obtained by using a metallocene catalyst among the Ziegler catalysts is excellent in mechanical strength such as tensile strength, tear strength or impact resistance but is inferior in melt tension, and is thus expected to exhibit a deterioration in moldability such as significant neck-in.
As an ethylene-based polymer excellent in moldability and mechanical strength, a composition comprising high-pressure low-densitypoly ethylene and an ethylene-based polymer obtained by using a metallocene catalyst is proposed for example in Japanese Patent Publication No. H06-65443 etc. However, when the content of high-pressure low-density polyethylene is low, the composition does not sufficiently improve melt tension, and is thus expected to exhibit a deterioration in moldability, such as significant neck-in in T-die molding. When the content of high-pressure low-density polyethylene is high, the composition is expected to be inferior in mechanical strength such as tensile strength, tear strength or impact resistance.
To solve the problem, various ethylene-based polymers having long branches introduced into them have been disclosed.
Japanese Patent Publication No. H02-276807 discloses an ethylene-based polymer obtained by solution polymerization in the presence of a catalyst consisting of ethylene bis(indenyl) hafnium dichloride and methyl alumoxane, Japanese Patent Publication No. H04-213309 discloses an ethylene-based polymer obtained by gaseous phase polymerization in the presence of a catalyst comprising ethylene bis(indenyl) zirconium dichloride and methyl alumoxane carried on silica, WO93/08221 discloses an ethylene-based polymer obtained by solution polymerization in the presence of a constraint geometrical catalyst, Japanese Patent Publication No. H08-311260 discloses an ethylene-based polymer obtained by gaseous phase polymerization in the presence of a catalyst comprising a racemate and mesoisomer of Me2Si(2-Me-Ind)2 and methyl alumoxane carried on silica, Japanese Patent Publication No. H08-34819 discloses an ethylene-based polymer obtained by slurry polymerization in the presence of a catalyst comprising bis(cyclopentadienyl) zirconium dichloride carried on chemically treated montmorilonite, and Japanese Patent Publication No. H08-319313 discloses an ethylene-based polymer obtained by polymerization using a catalyst consisting of Cp*Ti (OMe)3 and methyl alumoxane.
It is described that these ethylene-based polymers, as compared with long branch-free linear ethylene-based polymers, are excellent in moldability with improvement in melt tension, but neck-in is still significant so improvement in moldability is expected to be insufficient.
To improve moldability, an ethylene-based polymer having a small amount of a high-molecular component introduced into it is disclosed.
Japanese Patent Publication No. H06-172594 discloses an ethylene-based resin composition comprising a high-molecular component and a low-molecular component, obtained by using a Ziegler catalyst. This ethylene-based resin composition is expected to be inferior in moldability because of its too high molecular weight. Its compositional distribution is expected to be broad because of use of a Ziegler catalyst that is a multi-site catalyst.
Japanese Patent Publication No. H11-166083 discloses an ethylene-based resin composition comprising an ethylene-based polymer consisting of high- and low-molecular components obtained by using a Ziegler catalyst and an ethylene-based polymer obtained by using a Phillips catalyst. This ethylene-based resin composition is expected to be inferior in low-temperature sealability because of its too high density. Further the ethylene-based polymer obtained by using a Phillips catalyst is known to have many terminal vinyl groups. Accordingly, this ethylene-based resin composition is inferior in thermal stability. Further, its compositional distribution is expected to be broad because of use of a Ziegler catalyst that is a multi-site catalyst.
As described above, resin excellent in moldability and mechanical strength is hardly efficiently obtained on the basis of the conventional known techniques. In other words, if an ethylene-based polymer excellent in moldability and mechanical strength would appear, its industrial value would be extremely significant.
The present inventors made extensive study in view of these circumstances, and as a result, they found that an ethylene-based polymer excellent in high-speed film processability and mechanical strength with less neck-in in T-die molding is obtained by conferring a specific molecular structure and melt physical properties thereon, and the present invention was thereby completed.